Method of controlling a variable speed wind turbine generator

ABSTRACT

Methods of controlling a variable speed wind turbine generator connected to a power grid. The method may include measuring the frequency, f, of the power grid, controlling the speed of the generator for optimizing the power delivered to the power grid, and setting limits for the generator speed. The setting of the limits for the generator speed is performed in dependency of the measured frequency of the power grid. This provides a dynamical set of limits providing improved possibilities of optimizing the power production.

TECHNICAL FIELD

The present invention relates to variable speed wind turbine generators connected to the power grid and more specifically to the control of such wind turbine generators when the power grid frequency deviates from the nominal frequency.

BACKGROUND ART

Typically, such wind turbines have been controlled in generator speed to deliver maximum power to the power grid and certain fixed maximum and minimum speed limits have been imposed in order to limit the load on the turbine components, e.g. the converter, drive train, generator, etc.

U.S. Pat. No. 5,729,118 discloses a variable speed induction generator-motor with controllable excitation frequency, in which the slip frequency is kept constant during variations of the power grid frequency.

DISCLOSURE OF THE INVENTION

Based on this prior art it is the object of the present invention to provide a method of controlling a variable speed wind turbine generator with which it is possible to optimize the power generation by optimizing the generator speed. Preferably, the generator speed is controlled in dependency of the wind speed, and the control is performed by pitch control of the wind turbine blades. Furthermore, it is desirable to be able to set dynamical limits to the generator speed, said dynamical limits taking into account the possible variations in the power grid frequency, even at relatively large variations thereof. This object is achieved by a method of said kind, which according to the present invention also comprises the features set forth in the characterising part of claim 1. With this arrangement the limits for the speed of the generator are dynamically adapted to the frequency of the power grid, whereby the optimization of the power delivery from the generator is subjected to appropriate limits during periods of varying power grid frequencies. Preferred embodiments of the method, the advantages of which will be evident from the following detailed description of the invention, are revealed in the subordinate claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed part of the present description, the invention will be explained in more detail with reference to exemplary embodiments of a method according to the invention with reference to the drawing, in which

FIG. 1 schematically shows a diagram of the generator speed limits as a function of the deviation in grid frequency Δf in accordance with one preferred embodiment of the method, the deviation in grid grid frequency Δf being defined to be the measured grid frequency minus the nominal grid frequency, and

FIG. 2 schematically shows a corresponding diagram of the generator speed limit as a function of the deviation in grid frequency Δf in accordance with another preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As indicated above, the aim of the present invention is to perform a dynamic correction or adjustment of the upper and lower limits for the generator speed in accordance with deviations in the grid frequency. As indicated in the accompanying figures, such dynamical correction of the maximum and minimum speed for the generator can be performed in different intervals. As shown in FIG. 1, the maximum and minimum speed for the generator is set to fixed values, as long as the deviation from the nominal frequency is less than ±1%, corresponding to the interval 49.5 Hz to 50.5 Hz or 59.4 Hz to 60.6 Hz, respectively, for a 50 Hz and 60 Hz nominal grid frequency, respectively. The purpose of this deadband is to avoid corrections as long as the frequency deviations are insignificant and only perform corrections when the frequency deviations are extreme.

For the 50 Hz generator, typical values for the fixed values of the upper and lower limits would be synchronous speed, e.g. 1500 rpm, plus 12% and minus 16.7%, respectively. As can be seen in FIG. 1, the upper speed limit is furthermore fixedly limited to 1680 rpm in order to avoid overspeeding. This can be seen as the horizontal part of the upper limit above approx. 1.2 Hz deviation in grid frequency. Furthermore, as again seen in FIG. 1, this upper limit may be expanded below a deviation of approx. approx. −1.8 Hz to a generator speed of 1580 rpm. This fixation of the upper limit at approx. 1580 rpm is introduced in order to prevent the generator torque from increasing significantly at a negative deviation in the grid frequency.

For the lower speed limit, FIG. 1 indicates a constant percentage deviation from the synchronous speed calculated based on the deviating grid frequency Thus, the lower limit, apart from the interval from 49.5 to 50.5 Hz, is limited to the calculated synchronous speed based on the deviating grid frequency minus 16.7%.

In certain types of wind turbines, it is preferred to operate the speed of the generator below synchronous speed, i.e. operating sub-synchronously, at all times. The reason for this may be different, e.g. due to the fact that the possible back-to-back converter connected between the rotor windings and the grid, can be constructed to only deliver power in one direction.

In this situation, the upper and lower speed limits for the generator may be as shown in FIG. 2, where the rotational speed of the generator is kept below the calculated synchronous speed minus 2.3% for frequency deviations between −3.6 Hz and −0.6 Hz and kept below synchronous speed −2.3% for deviations in grid frequency between −0.6 Hz and +3.6 Hz. The lower limit for the generator speed is, as also shown in FIG. 1 above, −16.7% of calculated synchronous speed for the intervals of grid frequency deviation between −6% and −1% and between +1% and +6%, and nominal synchronous speed minus 16.7% at grid frequency deviation between −1% and +1%.

The stator of the generator can be connected to the power grid in two different configurations, star and delta configuration. Different restrictions apply to both speed and power in these two configurations, and examples of this can be seen in FIGS. 1 and 2, where, as typically found, the maximum speed limit will be identical for star and delta configuration, respectively, whereas the minimum speed limit will be lower for delta configuration than for star configuration, as indicated. 

1. A method of controlling a variable speed wind turbine generator connected to a power grid and comprising means for controlling the generator speed, said method: measuring the frequency f of the power grid, controlling the speed of the generator for optimizing the power delivered to the power grid, and setting limits for the generator speed, wherein the setting of the limits for the generator speed is performed in dependency of the measured frequency of the power grid.
 2. The method of claim 1, wherein the dependency is set as follows: a) whenever the measured frequency is within predetermined limits, a set of normal, fixed limits for the generator speed is imposed, and b) whenever the measured frequency is outside said predetermined limits, the speed limits for the generator are adjusted relative to said fixed limits.
 3. The method of claim 2, wherein the dependency, whenever the measured frequency is outside said predetermined limits, is an adjustment of the upper and lower speed limits with the same percentage as the frequency deviation Δf of the grid frequency from the nominal grid frequency.
 4. The method of claim 2, wherein said predetermined limits are provided as a percentage of deviation from the nominal frequency.
 5. The method of claim 4, wherein said percentage of deviation is ±1%.
 6. The method of claim 2, wherein the set of normal, fixed limits for the generator speed are set to provide an interval surrounding the synchronous speed at nominal frequency.
 7. The method of claim 1, wherein the generator speed is further limited by a fixed maximum generator speed.
 8. The method of claim 1, wherein the generator is disconnected from the power grid whenever the grid frequency exceed predetermined limits.
 9. The method of claim 1, wherein the generator is disconnected whenever the grid frequency deviates more than a predetermined percentage from the nominal frequency.
 10. The method of claim 4, wherein said percentage of deviation is ±6%.
 11. The method of claim 8, wherein the predetermined limits are a certain percentage deviation from the nominal frequency. 